The term “valve metal” represents a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating metal-oxide films upon anodic polarization in electrically conductive solutions. The performance of valve metal anodes in an actual capacitor depends upon several factors, e.g., the effective surface area of the anodes and cathodes that can be contacted by electrolyte, the dielectric constant of the oxide formed on the metal surface, the thickness of the oxide layer on top of the metal surface, the conductivity of the electrolyte, etc. The thickness of the anodic oxide layer is approximately proportional to the electrical potential applied to the anode during the formation of the anode (i.e., at the time when the anode is immersed into the formation electrolyte). For aluminum, the oxide grows approximately by ˜1.1 nm per Volt, for tantalum this “growth rate” is somewhat higher, approximately 1.8 nm per Volt. Niobium and tantalum anodes are typically made in the form of a pressed powder pellet or “slug” when used in an electrolytic capacitor.
The density of the pressed anode slug is typically significantly less than the density of the bulk metal of which the powder is made, i.e., up to ⅔ of the volume of a given slug may be open space (pore space). The final density of the anode slug is largely determined at the time of pressing, when a known amount of powder is pressed into a known volume. Traditionally, formation of the anode slug has been thought to require a fairly homogeneous distribution of open space throughout the anode slug since the forming electrolyte needs to wet even the most “remote” cavities in the karst-like internal structure of the anode. This is specifically important for comparatively large anodes with volumes of the order 1 cm3 or above.
In the context of medical devices, capacitors are typically charged and discharged rapidly for delivery of low voltage or high voltage stimuli. Upon or during detection of a potentially lethal arrhythmia, suitable electrical transformer circuitry charges one or more high voltage capacitors using a low voltage battery as a charge source. Then, at an appropriate time the energy stored in the capacitor(s) discharges through a pair of electrodes disposed in or near a patient's heart. The discharged energy is used to terminate the arrhythmia and restore organized cardiac activity. Medical devices that deliver cardioversion and/or defibrillation therapy include automated external defibrillators (AEDs) and implantable cardioverter-defibrillators (ICDs). For purposes of the present invention, an ICD is understood to encompass all such IMDs having at least high voltage cardioversion or defibrillation capabilities. In most all IMDs, energy, volume, thickness and mass are critical features. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the effective cardioversion/defibrillation therapy have historically been relatively bulky and expensive. Therefore, the industry has been working towards smaller battery and capacitor volumes. A high capacitance cathode, such as the one proposed herein, contributes to these on-going efforts.